1. Field of the Invention
The present invention relates to a photomask, and more particularly, to a photomask for measuring lens aberration, a method of manufacturing the photomask, and a method of measuring lens aberration using the photomask.
2. Description of the Related Art
Circuit patterns that constitute a semiconductor device or a display device are formed by photolithography in an exposing apparatus. The exposure apparatus forms a circuit pattern on a wafer by radiating light onto a photomask. The exposure apparatus has an optical system that includes a lens to project the shape of a reduced shielding pattern of the photomask by collecting light on a surface of the wafer.
However, the lens in the optical system generally has an aberration that causes an imperfect focus on the wafer. The lens aberration may change the location of a circuit pattern on a wafer, which may cause various problems, such as a short-circuit on a highly integrated semiconductor device, which may have little or no space margin. Therefore, it is important to measure the lens aberration before performing a photolithography process to form a circuit pattern. Various methods for measuring the lens aberration have been proposed.
The prevailing method of measuring the lens aberration is to measure the magnitude of variation of an overlay value after measuring the overlay values using a lens aberration measuring photomask having a shielding pattern. More specifically, as depicted in FIG. 1, a wafer 40, on which a photoresist film 45 is formed, is loaded into an exposure apparatus (not shown) that includes an optical system composed of lenses 32 and 33 that are to be measured. Next, a reference photoresist pattern 46 is formed on the wafer by radiating light 50 onto a first photomask 10 after aligning the first photomask 10 on which a reference pattern 12 is formed. Afterward, an encoded photoresist pattern 47 is formed on the wafer 40 by irradiating light onto a second photomask 20 after replacing the first photomask 10 with the second photomask 20 on which an encoded pattern 22 is formed.
Next, an overlay value is obtained by measuring the distances ×1 and ×2 between the reference photoresist pattern 46 and the encoded photoresist pattern 47, and the lens aberration is measured based on the overlay value.
At this time, an aperture 15 is included to verify the lens aberration at a specific region of a lens pupil between the photomask 10 or 20 and a lens 30. The aperture 15 includes at least one pin hole 17 for confining the angle of light irradiated onto the lens 30.
Also, the wafer 40 must be located outside the optimal focus F to form a plurality of reference patterns 12 and encoded patterns 22 on predetermined regions on the wafer 40.
However, the above method of measuring lens aberration requires two photomasks 10 and 20. Therefore, it takes time to replace the first photomask 10 with the second photomask 20, thereby increasing the overall process time.
Also, an additional aligning process is required whenever the first photomask 10 is replaced with the second photomask 20. Furthermore, if the second photomask 20 is misaligned, an overlay variation may occur regardless of the lens aberration, which makes the correct measurement of lens aberration difficult.
Also, a high exposure energy of several J/cm2 is required to form the photoresist patterns 46 and 47, using a light source having, for example, a wavelength of 248 nm or 193 nm, since the light irradiated through the photomasks 10 and 20 is transferred to the photoresist film 45 through a small pin hole having a diameter of a few tens of μm in the aperture 15 or 25, thereby reducing the exposure efficiency. For reference, an exposure energy required for forming a photoresist pattern is generally 20-30 mJ/cm2.